Power steering fluid

ABSTRACT

Provided are power steering fluids made from a waxy feed and with improved low temperature properties, such as, for example, a viscosity index of greater than 290 and a Brookfield Viscosity at −40° C. of less than 1900 mPa·s. In an embodiment, the power steering fluid comprises greater than 50 weight % base oil, viscosity index improver, and less than about 1.0 weight % pour point depressant.

FIELD OF ART

Provided are power steering fluids with improved low temperature properties and high viscosity index, and more specifically, power steering fluids made from a base oil having consecutive numbers of carbon atoms and having a very high viscosity index.

BACKGROUND

Power steering fluids are an integral part of all power steering systems. Power steering fluid is used in about 80 to 90% of all vehicles in North America and Japan, as well as increasing numbers of vehicles in other parts of the world. Original Equipment Manufacturers have stringent specifications for power steering fluids. Requirements include high oxidation stability, high viscosity index, and compatibility with seals and hoses. In the past, power steering fluids used blends of naphthenic and solvent neutral base oils. Newer power steering fluids have been formulated with blends of naphthenic, solvent neutral, and hydrocracked base stocks. The hydrocracked base oils used in power steering fluids have had saturates contents of about 90 to about 99 mass %. Power steering fluids with improved viscosity index and lower Brookfield viscosity are needed.

SUMMARY

Provided is a power steering fluid comprising greater than 50 weight % base oil and viscosity index improver and having a viscosity index of greater than 290 and a Brookfield Viscosity at −40° C. of less than 1900 mPa·s. The base oil has consecutive numbers of carbon atoms and has a viscosity index greater than a viscosity index calculated by the following equation:

28×ln(Kinematic Viscosity at 100° C.)+101;

Also provided is a process for producing a power steering fluid comprising greater than 50 weight % base oil and having a viscosity index of greater than 290 and a Brookfield Viscosity at −40° C. of less than 1900 mPa·s, the process comprising obtaining a base oil and blending the base oil with viscosity index improver to form the power steering fluid. The base oil has consecutive numbers of carbon atoms; a kinematic viscosity at 100° C. of less than about 4 mm²/s; and a Noack volatility less than a Noack Volatility Factor calculated by the following equation:

160−40(Kinematic Viscosity at 100° C.).

BRIEF DESCRIPTION OF THE FIGURES OF THE DRAWING

FIG. 1 is a graph of the Viscosity Index Factor calculated by the following equation:

Viscosity Index Factor=28×ln(Kinematic Viscosity at 100° C.)+101.   (1)

FIG. 2 is a graph of the Noack Volatility Factor calculated by the following equations:

Noack Volatility Factor=160−40(Kinematic Viscosity at 100° C.);   (2)

Noack Volatility Factor=(900×(Kinematic Viscosity at 100° C.)^(−2.8))−15.   (3)

DETAILED DESCRIPTION

In an embodiment, the base oil of the power steering fluid has a kinematic viscosity at 100° C. between about 1.2 mm²/s and less than about 4.0 mm²/s, a high viscosity index, low Cold Cranking Simulator (CCS) viscosity (less than 1500 mPa·s at −35° C.), and in an embodiment, cycloparaffin composition of greater than 5 weight % total molecules with cycloparaffinic functionality, and a ratio of molecules with monocycloparaffinic functionality to molecules with multicycloparaffinic functionality of greater than 2.1, such as greater than 5, greater than 10, greater than 15, or greater than 20.

Polyalphaolefin (PAO) oils are an oligomerization product of even carbon numbered linear alpha olefins, typically 1-decene. The PAO oil molecules, therefore, comprise a mixture of even carbon numbered hydrocarbon molecules, differing from each other in the number of carbon atoms, by multiples of the number of carbon atoms in the linear alpha olefin starting monomer. Even if a mixture of linear alpha olefin monomers having even numbers of carbon atoms (e.g., decene and dodecene) were oligomerized to form a heavy lubricant base stock oil, the number of carbon atoms in the resulting hydrocarbon molecules would still have even numbers of carbon atoms. This is different from the mixture of consecutive numbers of carbon atoms in the hydrocarbon molecules of the base oil of the power steering fluid disclosed herein, which comprise hydrocarbon molecules having both even and odd numbers of carbon atoms and which differ from each other by consecutive numbers of carbon atoms (e.g., 1, 2, 3, 4, 5, 6, 7 and more carbon atoms).

The phrase “consecutive numbers of carbon atoms” means that the base oil has a distribution of hydrocarbon molecules over a range of carbon numbers, with every number of carbon numbers in-between. For example, the base oil may have hydrocarbon molecules ranging from C₂₂ to C₃₆ or from C₃₀ to C₆₀ with every carbon number in-between. The hydrocarbon molecules of the base oil of the power steering fluid differ from each other by consecutive numbers of carbon atoms, as a consequence of the waxy feed also having sequential numbers of carbon atoms. For example, in the Fischer-Tropsch hydrocarbon synthesis reaction the source of carbon atoms is CO and the hydrocarbon molecules are built up one carbon atom at a time. Petroleum-derived waxy feeds also have sequential numbers of carbon numbers. In contrast to an oil based on PAO, the molecules of the base oil of the power steering fluid disclosed herein have a more linear structure, comprising a relatively long backbone with short branches. The classic textbook description of a PAO is a star-shaped molecule, and in particular tridecane, which is illustrated as three decane molecules attached at a central point. While a star-shaped molecule is theoretical, nevertheless PAO molecules have fewer and longer branches than the hydrocarbon molecules that make up the base oil of the power steering fluid.

In an embodiment, the base oil of the power steering fluid comprises consecutive numbers of carbon atoms. The power steering fluid comprises greater than 50 weight % base oil; and viscosity index improver in an amount less than 13 weight %, or less than 12 weight %. The power steering fluid comprises between 0 and less than about 1.0 weight % pour point depressant. The power steering fluid has a Brookfield Viscosity at −40° C. of less than 1900 mPa·s. The power steering fluid, in an embodiment, comprises a detergent-inhibitor additive package, for example, 2 to 6 weight % or 5 weight % detergent-inhibitor additive package.

In an embodiment, the power steering fluids (e.g., for automobiles and light trucks) meet the requirements of a variety of specifications for power steering fluids used in automotive power steering systems or are suitable for power steering fluid service fill replacement. Examples of power steering fluid specifications are: DaimlerChrysler MS5931F, DaimlerChrysler MS1872, Ford M2C138-CJ, Ford M2C33-F, Ford ESW-M2C128-C & D, GM 9985010, Navistar TMS 6810, and Volkswagen TL-VW-570-26. Examples of power steering fluid part numbers are: Acura/Honda Part Number 08206-9002 PE, Audi Part Number G002000, Mercedes Benz Part Number 00 989 8803, Saab Part Number 30 09 800, and Subaru Part Number K0Z09A0080.

Fischer-Tropsch derived base oils produced by catalytic hydroisomerization have excellent oxidation stability, low volatility, and high viscosity index. Fischer-Tropsch derived base oils contain greater than 95 weight % or greater than 99.0 weight %, or greater than 99.5 weight % saturates, which in addition distinguishes them from most hydrocracked base oils used previously in power steering fluids. Because of their good properties, Fischer-Tropsch derived base oils with viscosities between about 1.2 and about 4.0 mm²/s at 100° C. can be blended into power steering fluids. Fischer-Tropsch derived base oils have inherently good lubricant characteristics, due to their content of molecules with cycloparaffinic functionality, and therefore have natural lubricity, wear resistance, solvency and seal compatibility. Fischer-Tropsch derived base oils also are fully compatible with naphthenic and solvent neutral base oils, and when combined with other types of base oils make a base oil blend that is further enhanced in the aforementioned lubricant characteristics, especially, in their compatibility with the elastomers in the seals and hoses of power steering systems.

Definitions and Terms

The following terms will be used throughout the specification and will have the following meanings unless otherwise indicated.

The term “Fischer-Tropsch derived” means that the product, fraction, or feed originates from or is produced at some stage by a Fischer-Tropsch process.

The term “petroleum derived” means that the product, fraction, or feed originates from the vapor overhead streams from distilling petroleum crude and the residual fuels that are the non-vaporizable remaining portion. A source of the petroleum derived product, fraction, or feed can be from a gas field condensate.

Highly paraffinic wax means a wax having a high content of n-paraffins, generally greater than 40 weight %, but can be greater than 50 weight %, or even greater than 75 weight %. In an embodiment, the highly paraffinic waxes also have very low levels of nitrogen and sulfur, generally less than 25 ppm total combined nitrogen and sulfur, for example, less than 20 ppm. Examples of highly paraffinic waxes include slack waxes, deoiled slack waxes, refined foots oils, waxy lubricant raffinates, n-paraffin waxes, NAO waxes, waxes produced in chemical plant processes, deoiled petroleum derived waxes, microcrystalline waxes, Fischer-Tropsch waxes, and mixtures thereof. In an embodiment, the pour points of the highly paraffinic waxes are greater than 50° C. or greater than 60° C.

The term “derived from highly paraffinic wax” means that the product, fraction, or feed originates from or is produced at some stage by from a highly paraffinic wax.

Aromatics means any hydrocarbonaceous compounds that contain at least one group of atoms that share an uninterrupted cloud of delocalized electrons, where the number of delocalized electrons in the group of atoms corresponds to a solution to the Huckel rule of 4n+2 (e.g., n=1 for 6 electrons, etc.). Representative examples include, but are not limited to, benzene, biphenyl, naphthalene, and the like.

Molecules with cycloparaffinic functionality mean any molecule that is, or contains as one or more substituents, a monocyclic or a fused multicyclic saturated hydrocarbon group. The cycloparaffinic group can be optionally substituted with one or more, such as one to three, substituents. Representative examples include, but are not limited to, cyclopropyl, cyclobutyl, cyclohexyl, cyclopentyl, cycloheptyl, decahydronaphthalene, octahydropentalene, (pentadecan-6-yl)cyclohexane, 3,7,10-tricyclohexylpentadecane, decahydro-1-(pentadecan-6-yl)naphthalene, and the like.

Molecules with monocycloparaffinic functionality mean any molecule that is a monocyclic saturated hydrocarbon group of three to seven ring carbons or any molecule that is substituted with a single monocyclic saturated hydrocarbon group of three to seven ring carbons. The cycloparaffinic group can be optionally substituted with one or more, such as one to three, substituents. Representative examples include, but are not limited to, cyclopropyl, cyclobutyl, cyclohexyl, cyclopentyl, cycloheptyl, (pentadecan-6-yl)cyclohexane, and the like.

Molecules with multicycloparaffinic functionality mean any molecule that is fused multicyclic saturated hydrocarbon ring group of two or more fused rings, any molecule that is substituted with one or more fused multicyclic saturated hydrocarbon ring groups of two or more fused rings, or any molecule that is substituted with more than one monocyclic saturated hydrocarbon group of three to seven ring carbons. The fused multicyclic saturated hydrocarbon ring group often is of two fused rings. The cycloparaffinic group can be optionally substituted with one or more, such as one to three, substituents. Representative examples include, but are not limited to, decahydronaphthalene, octahydropentalene, 3,7,10-tricyclohexylpentadecane, decahydro-1-(pentadecan-6-yl)naphthalene, and the like.

Brookfield Viscosity: ASTM D2983-04a is used to determine the low-shear-rate viscosity of automotive fluid lubricants at low temperatures. The low-temperature, low-shear-rate viscosity of automatic transmission fluids, gear oils, torque and tractor fluids, and industrial and automotive hydraulic oils are frequently specified by Brookfield viscosities.

Kinematic viscosity is a measurement of the resistance to flow of a fluid under gravity. Many base oils, power steering fluids made from them, and the correct operation of equipment depends upon the appropriate viscosity of the fluid being used. Kinematic viscosity is determined by ASTM D445-06. The results are reported in mm²/s. In an embodiment, Fischer-Tropsch derived base oil has a kinematic viscosity of between about 1.2 mm²/s and about 4.0 mm²/s at 100° C. In an embodiment, base oil derived from highly paraffinic wax has a kinematic viscosity of between about 1.5 mm²/s and about 3.5 mm²/s. In an embodiment, base oil derived from high paraffinic wax has a kinematic viscosity of between about 2.0 mm²/s and about 3.5 mm²/s at 100° C., and in an embodiment, base oil derived from highly paraffinic wax has a kinematic viscosity of between about 2.0 mm²/s and about 3.0 mm²/s at 100° C.

Viscosity index (VI) is an empirical, unitless number indicating the effect of temperature change on the kinematic viscosity of the oil. Viscosity index is determined by ASTM D2270-04. In an embodiment, base oil derived from highly paraffinic wax has a viscosity index of greater than 101. In an embodiment, base oil derived from highly paraffinic wax has a viscosity index of between about 105 and about 160.

The “Viscosity Index Factor” of base oil derived from highly paraffinic wax is an empirical number derived from kinematic viscosity of the base oil. The Viscosity Index Factor is calculated by the following equation:

Viscosity Index Factor=28×ln(Kinematic Viscosity at 100° C.)+101   (1)

wherein “ln” is the logarithm function to the base “e”.

Base oil derived from highly paraffinic wax can have a viscosity index greater than the Viscosity Index Factor. FIG. 1 is a graph of the Viscosity Index Factor according to the above equation. Earlier base oils derived from highly paraffinic wax having high viscosity indexes derived from kinematic viscosity of the base oil, such as those taught in U.S. Pat. No. 7,083,713, are also useful in power steering fluids. However, the base oils useful in the compositions of the power steering fluid disclosed herein have viscosity indexes that are higher than the viscosity indexes of those taught in U.S. Pat. No. 7,083,713. The higher viscosity index of the base oil (Viscosity Index Factor) of the power steering fluid disclosed herein contributes to the improved properties (high viscosity index and low Brookfield Viscosity) of the power steering fluid.

Pour point is a measurement of the temperature at which a sample of base oil will begin to flow under carefully controlled conditions. Pour point can be determined as described in ASTM D5950-02. The results are reported in degrees Celsius. Many commercial base oils have specifications for pour point. When base oils have low pour points, the base oils are also likely to have other good low temperature properties, such as low cloud point, low cold filter plugging point, and low temperature cranking viscosity

Noack volatility is usually tested according to ASTM D5800-05 Procedure B. A more convenient method for calculating Noack volatility and one which correlates well with ASTM D5800-05 is by using a thermogravimetric analyzer (TGA) test by ASTM D6375-05. In an embodiment, base oil derived from highly paraffinic wax, has a Noack volatility of less than 100 weight %. Noack volatility of base oils generally increases as the kinematic viscosity decreases. The lower the Noack volatility, the lower the tendency of base oil and formulated oils to volatilize in service. The “Noack Volatility Factor” of base oil is an empirical number derived from the kinematic viscosity of the base oil. The Noack volatility of the base oil derived from highly paraffinic wax is very low, and in an embodiment, is less than an amount calculated by the equation:

Noack Volatility Factor=160−40(Kinematic Viscosity at 100° C.).   (2)

Equation (2), as provided in U.S. Patent Application Publication No. 2006/0201852 A1, provides Noack Volatility Factors between 0 and 100 for kinematic viscosities between 1.5 and 4.0 mm²/s. FIG. 2 is a graph of the Noack Volatility Factor according to Equation (2). In an embodiment, the Noack volatility of the base oil derived from highly paraffinic wax is less than an amount calculated by the equation:

Noack Volatility Factor=(900×(Kinematic Viscosity at 100° C.)^(−2.8))−15.   (3)

Equation (3), as provided in U.S. patent application Ser. No. 11/613,936, provides Noack Volatility Factors between 0 and 100 for kinematic viscosities between 2.09 and 4.3 mm²/s. FIG. 2 also includes the Noack Volatility Factor according to Equation (3). For kinematic viscosities in the range of 2.4 to 3.8 mm²/s, Equation (3) provides a lower Noack Volatility Factor than does Equation (2). Lower Noack Volatility Factors in the range of base oils having kinematic viscosities from 2.4 to 3.8 mm²/s are desired, especially if the base oils are to be blended with other oils that may have higher Noack volatilities.

The aniline point test indicates if an oil is likely to damage elastomers (rubber compounds) that come in contact with the oil. The aniline point is called the “aniline point temperature”, which is the lowest temperature (° F. or ° C.) at which equal volumes of aniline (C₆H₅NH₂) and the oil form a single phase. The aniline point is determined by ASTM D611-04. In an embodiment, the base oil of the power steering fluid, derived from highly paraffinic wax, have an aniline point greater than 36×ln(Kinematic Viscosity at 100° C.)+200. Accordingly, base oil derived from highly paraffinic wax exhibits good elastomer compatibility, and performs well with the seals and hoses in power steering systems.

Highly Paraffinic Wax

The highly paraffinic wax used in making the base oil of the power steering fluid can be any wax having a high content of n-paraffins and having consecutive numbers of carbon atoms. The highly paraffinic wax comprises greater than 40 weight % n-paraffins, such as greater than 50 weight %, or greater than 75 weight %. In an embodiment, the highly paraffinic waxes also have very low levels of nitrogen and sulfur, generally less than 25 ppm total combined nitrogen and sulfur, for example less than 20 ppm. Examples of highly paraffinic waxes include slack waxes, deoiled slack waxes, refined foots oils, waxy lubricant raffinates, n-paraffin waxes, NAO waxes, waxes produced in chemical plant processes, deoiled petroleum derived waxes, microcrystalline waxes, Fischer-Tropsch waxes, and mixtures thereof. In an embodiment, the pour points of the highly paraffinic waxes are greater than 50° C. or greater than 60° C.

It has been discovered that highly paraffinic waxes can be processed to provide base oil having low volatility, high viscosity index, and also having good additive solubility and elastomer compatibility. In an embodiment, the highly paraffinic wax is a Fischer-Tropsch derived wax and provides a Fischer-Tropsch derived base oil.

Fischer-Tropsch Synthesis

In Fischer-Tropsch chemistry, hydrogen and carbon monoxide is converted to liquid and gaseous hydrocarbons by contact with a Fischer-Tropsch catalyst under reactive conditions. Examples of conditions for performing Fischer-Tropsch type reactions are well known to those of skill in the art.

The Fischer-Tropsch synthesis products can be obtained by well-known processes such as, for example, the commercial SASOL® Slurry Phase Fischer-Tropsch technology, the commercial SHELL® Middle Distillate Synthesis (SMDS) Process, or by the non-commercial EXXON® Advanced Gas Conversion (AGC-21) process. Details of these processes and others are described in, for example, EP-A-776959, EP-A-668342, EP-B-450860; U.S. Pat. Nos. 4,943,672, 5,059,299, 5,348,982, 5,733,839 and RE39073; U.S. Application Publication No. 2005/0227866, WO-A-9934917, WO-A-9920720 and WO-A-05107935. The Fischer-Tropsch synthesis product usually comprises hydrocarbons having 1 to 100, or even more than 100 carbon atoms, and typically includes paraffins, olefins and oxygenated products.

The slurry Fischer-Tropsch process utilizes superior heat (and mass) transfer characteristics for the strongly exothermic synthesis reaction and is able to produce relatively high molecular weight, paraffinic hydrocarbons when using a cobalt catalyst.

Certain Fischer-Tropsch catalysts are known to provide relatively high chain growth probabilities, and the reaction products include a relatively low proportion of low molecular (C₂₋₈) weight olefins and a relatively high proportion of high molecular weight (C₃₀₊) waxes. Such catalysts are well known to those of skill in the art and can be readily obtained and/or prepared.

The product from a Fischer-Tropsch process contains predominantly paraffins. The products from Fischer-Tropsch reactions generally include a light reaction product and a waxy reaction product. The waxy reaction product (i.e., the waxy fraction) includes hydrocarbons boiling above about 600° F. (e.g., vacuum gas oil through heavy paraffins), largely in the C₂₀₊ range, with decreasing amounts down to C₁₀.

The waxy reaction product generally comprises greater than 70 weight % normal paraffins, and often greater than 80 weight % normal paraffins. It is the waxy reaction product (i.e., the waxy fraction) that is used as a feedstock to the process for providing Fischer-Tropsch derived base oil in power steering fluids.

The Fischer-Tropsch base oil of the power steering fluid can be prepared from the waxy fractions of the Fischer-Tropsch syncrude by a process including hydroisomerization. In an embodiment, the Fischer-Tropsch base oils are made by a process as described in U.S. Patent Application Publication Nos. 2005/0133409 A1 and 2006/0289337 A1. The Fischer-Tropsch base oil of the power steering fluid is often manufactured at a site different from the site at which the components of the power steering fluids are received and blended.

Process for Providing Base Oil

In an embodiment, the base oil of the power steering fluid is made by a process comprising providing a highly paraffinic wax and then hydroisomerizing the highly paraffinic wax to provide the base oil. The highly paraffinic wax is hydroisomerized using a shape selective intermediate pore size molecular sieve comprising a noble metal hydrogenation component under conditions of about 600° F. to 750° F.

In an embodiment, the highly paraffinic wax is a Fischer-Tropsch derived wax and provides a Fischer-Tropsch derived base oil. Fischer-Tropsch derived base oil can be made by a Fischer-Tropsch synthesis process followed by hydroisomerization of the waxy fractions of the Fischer-Tropsch syncrude.

Hydroisomerization

The highly paraffinic waxes are subjected to a process comprising hydroisomerization to provide the base oil of the power steering fluid. Hydroisomerization is intended to improve the cold flow properties of the base oil by the selective addition of branching into the molecular structure. Hydroisomerization ideally will achieve high conversion levels of the highly paraffinic wax to non-waxy iso-paraffins while at the same time minimizing the conversion by cracking. In an embodiment, the conditions for hydroisomerization are controlled such that the conversion of the compounds boiling above about 700° F. in the waxy feed to compounds boiling below about 700° F. is maintained between about 10 and 50 weight %, for example between 15 and 45 weight %.

Hydroisomerization is conducted using a shape selective intermediate pore size molecular sieve. The hydroisomerization catalysts used comprise a shape selective intermediate pore size molecular sieve and optionally a catalytically active metal hydrogenation component on a refractory oxide support. The phrase “intermediate pore size”, as used herein, means an effective pore aperture in the range of from about 3.9 to about 7.1 Å when the porous inorganic oxide is in the calcined form. The shape selective intermediate pore size molecular sieves used are generally 1-D 10-, 11- or 12-ring molecular sieves. In an embodiment, the molecular sieves are of the 1-D 10-ring variety, where 10- (or 11- or 12-) ring molecular sieves have 10 (or 11 or 12) tetrahedrally-coordinated atoms (T-atoms) joined by oxygens. In the 1-D molecular sieve, the 10-ring (or larger) pores are parallel with each other, and do not interconnect. Note, however, that 1-D 10-ring molecular sieves which meet the broader definition of the intermediate pore size molecular sieve but include intersecting pores having 8-membered rings can also be encompassed within the definition of molecular sieve. The classification of intrazeolite channels as 1-D, 2-D and 3-D is set forth by R. M. Barrer in Zeolites, Science and Technology, edited by F. R. Rodrigues, L. D. Rollman and C. Naccache, NATO ASI Series, 1984 which classification is incorporated in its entirety by reference (see particularly page 75).

Other shape selective intermediate pore size molecular sieves used for hydroisomerization are based upon aluminum phosphates, such as SAPO-11, SAPO-31, and SAPO-41. SM-3 is an example of a good shape selective intermediate pore size SAPO, which has a crystalline structure falling within that of the SAPO-11 molecular sieves. The preparation of SM-3 and its unique characteristics are described in U.S. Pat. Nos. 4,943,424 and 5,158,665. Other shape selective intermediate pore size molecular sieves used for hydroisomerization are zeolites, such as ZSM-22, ZSM-23, ZSM-35, ZSM-48, ZSM-57, SSZ-32, offretite, and ferrierite.

In an embodiment, an intermediate pore size molecular sieve is characterized by selected crystallographic free diameters of the channels, selected crystallite size (corresponding to selected channel length), and selected acidity. Desirable crystallographic free diameters of the channels of the molecular sieves are in the range of from about 3.9 to about 7.1 Å, having a maximum crystallographic free diameter of not more than 7.1 and a minimum crystallographic free diameter of not less than 3.9 Å. In this embodiment, the maximum crystallographic free diameter is not more than 7.1 and the minimum crystallographic free diameter is not less than 4.0 Å. In an embodiment, the maximum crystallographic free diameter is not more than 6.5 and the minimum crystallographic free diameter is not less than 4.0 Å. The crystallographic free diameters of the channels of molecular sieves are published in the “Atlas of Zeolite Framework Types”, Fifth Revised Edition, 2001, by Ch. Baerlocher, W. M. Meier, and D. H. Olson, Elsevier, pp 10-15.

An example of an intermediate pore size molecular sieve is described, for example, in U.S. Pat. Nos. 5,135,638 and 5,282,958. In U.S. Pat. No. 5,282,958, such an intermediate pore size molecular sieve has a crystallite size of no more than about 0.5 microns and pores with a minimum diameter of at least about 4.8 Å and with a maximum diameter of about 7.1 Å. The catalyst has sufficient acidity so that 0.5 grams thereof when positioned in a tube reactor converts at least 50% of hexadecane at 370° C., a pressure of 1200 psig, a hydrogen flow of 160 ml/min, and a feed rate of 1 ml/hr. The catalyst also exhibits isomerization selectivity of 40 percent or greater (isomerization selectivity is determined as follows: 100×(weight % branched C₁₆ in product)/(weight % branched C₁₆ in product+weight % C¹³⁻ in product) when used under conditions leading to 96% conversion of normal hexadecane (n-C₁₆) to other species.

In an embodiment, the molecular sieve can further be characterized by pores or channels having a crystallographic free diameter in the range of from about 4.0 to about 7.1 Å, for example, in the range of 4.0 to 6.5 Å. The crystallographic free diameters of the channels of molecular sieves are published in the “Atlas of Zeolite Framework Types”, Fifth Revised Edition, 2001, by Ch. Baerlocher, W. M. Meier, and D. H. Olson, Elsevier, pp 10-15.

If the crystallographic free diameters of the channels of a molecular sieve are unknown, the effective pore size of the molecular sieve can be measured using standard adsorption techniques and hydrocarbonaceous compounds of known minimum kinetic diameters. See Breck, Zeolite Molecular Sieves, 1974 (especially Chapter 8); Anderson et al. J. Catalysis 58, 114 (1979); and U.S. Pat. No. 4,440,871. In performing adsorption measurements to determine pore size, standard techniques are used. It is convenient to consider a particular molecule as excluded if does not reach at least 95% of its equilibrium adsorption value on the molecular sieve in less than about 10 minutes (p/p_(o)=0.5 at 25° C.). Intermediate pore size molecular sieves will typically admit molecules having kinetic diameters of 5.3 to 6.5 Å with little hindrance.

Hydroisomerization catalysts often comprise a catalytically active hydrogenation metal. The presence of a catalytically active hydrogenation metal leads to product improvement, especially viscosity index and stability. Typical catalytically active hydrogenation metals include chromium, molybdenum, nickel, vanadium, cobalt, tungsten, zinc, platinum, and palladium. In an embodiment the catalytically active hydrogen metals are selected from platinum, palladium, and mixtures thereof If platinum and/or palladium is used, the total amount of active hydrogenation metal is typically in the range of 0.1 to 5 weight percent of the total catalyst, usually from 0.1 to 2 weight percent, and not to exceed 10 weight percent.

The refractory oxide support can be selected from those oxide supports, which are conventionally used for catalysts, including silica, alumina, silica-alumina, magnesia, titania and combinations thereof.

The conditions for hydroisomerization will be tailored to achieve a base oil comprising greater than 5 weight % molecules with cycloparaffinic functionality. In an embodiment, the conditions provide a base oil comprising a ratio of weight percent of molecules with monocycloparaffinic functionality of weight percent of molecules with multicycloparaffinic functionality of greater than 5, such as greater than 10, greater than 15, or greater than 20. The conditions for hydroisomerization will depend on the properties of feed used, the catalyst used, whether or not the catalyst is sulfided, the desired yield, and the desired properties of the base oil. Conditions under which the hydroisomerization process can be carried out include temperatures from about 500° F. to about 775° F. (260° C. to about 413° C.), such as 600° F. to about 750° F. (315° C. to about 399° C.), or 600° F. to about 700° F. (315° C. to about 371° C.); and pressures from about 15 to 3000 psig, such as 100 to 2500 psig. The hydroisomerization pressures in this context refer to the hydrogen partial pressure within the hydroisomerization reactor, although the hydrogen partial pressure is substantially the same (or nearly the same) as the total pressure. The liquid hourly space velocity during contacting is generally from about 0.1 to 20 hr⁻¹, for example, from about 0.1 to about 5 hr⁻¹. The hydrogen to hydrocarbon ratio falls within a range from about 1.0 to about 50 moles H₂ per mole hydrocarbon, for example, from about 10 to about 20 moles H₂ per mole hydrocarbon. Suitable conditions for performing hydroisomerization are described in U.S. Pat. Nos. 5,282,958 and 5,135,638.

Hydrogen is present in the reaction zone during the hydroisomerization process, typically in a hydrogen to feed ratio from about 0.5 to 30 MSCF/bbl (thousand standard cubic feet per barrel), such as from about 1 to about 10 MSCF/bbl. In an embodiment, the hydrogen to feed ratio is from about 712.4 to about 3562 liter H₂/liter oil (about 4 to about 20 MSCF/bbl), Hydrogen will sometimes be separated from the product and recycled to the reaction zone.

Hydrotreating

The highly paraffinic waxy feed to the hydroisomerization process will sometimes be hydrotreated prior to hydroisomerization. Hydrotreating refers to a catalytic process, usually carried out in the presence of free hydrogen, in which the primary purpose is the removal of various metal contaminants, such as arsenic, aluminum, and cobalt; heteroatoms, such as sulfur and nitrogen; oxygenates; or aromatics from the feed stock. Generally, in hydrotreating operations cracking of the hydrocarbon molecules, i.e., breaking the larger hydrocarbon molecules into smaller hydrocarbon molecules, is minimized, and the unsaturated hydrocarbons are either fully or partially hydrogenated.

Hydrofinishing

Hydrofinishing is a hydrotreating process that will often be used as a step following hydroisomerization to provide base oil derived from highly paraffinic wax. Hydrofinishing is intended to improve oxidation stability, UV stability, and appearance of base oil by removing traces of aromatics, olefins, color bodies, and solvents. As used herein, the term UV stability refers to the stability of base oil or power steering fluids when exposed to UV light and oxygen. Instability is indicated when a visible precipitate forms, usually seen as floc or cloudiness, or a darker color develops upon exposure to ultraviolet light and air. A general description of hydrofinishing can be found in U.S. Pat. Nos. 3,852,207 and 4,673,487. Clay treating to remove impurities is an alternative final process step to provide base oil derived from highly paraffinic wax.

Fractionation

Optionally, the process to provide the light base oil derived from highly paraffinic wax can include fractionating the highly paraffinic waxy feed prior to hydroisomerization, or fractionating of base oil obtained from the hydroisomerization process. The fractionation of the highly paraffinic waxy feed or the isomerized base oil into fractions is generally accomplished by either atmospheric or vacuum distillation, or by a combination of atmospheric and vacuum distillation. Atmospheric distillation is typically used to separate the lighter distillate fractions, such as naphtha and middle distillates, from a bottoms fraction having an initial boiling point above about 600° F. to about 750° F. (about 315° C. to about 399° C.). At higher temperatures thermal cracking of the hydrocarbons can take place leading to fouling of the equipment and to lower yields of the heavier cuts. Vacuum distillation is typically used to separate the higher boiling material, such as base oil, into different boiling range cuts. Fractionating base oil into different boiling range cuts enables base oil manufacturing plant to produce more than one grade, or viscosity, of base oil.

Solvent Dewaxing

The process to make base oil derived from highly paraffinic wax will sometimes also includes a solvent dewaxing step either before or following the hydroisomerization process. Solvent dewaxing optionally can be used to remove small amounts of remaining waxy molecules from base oil after hydroisomerization. Solvent dewaxing is done by dissolving base oil in a solvent, such as methyl ethyl ketone, methyl iso-butyl ketone, or toluene, or precipitating the wax molecules as discussed in Chemical Technology of Petroleum, 3rd Edition, William Gruse and Donald Stevens, McGraw-Hill Book Company, Inc., New York, 1960, pages 566 to 570. Solvent dewaxing is also described in U.S. Pat. Nos. 4,477,333, 3,773,650 and 3,775,288.

Base Oil Derived from Highly Paraffinic Wax

Base oil derived from highly paraffinic wax is suitable for use in power steering fluids. In an embodiment, base oil derived from highly paraffinic wax has a viscosity of between about 1.2 mm²/s and about 4.0 mm²/s, such as between about 1.5 and about 3.5 mm²/s at 100° C., or between about 2 mm²/s and about 3.5 mm²/s at 100° C., or between about 2 mm²/s and about 3.0 mm²/s at 100° C. Base oil derived from highly paraffinic wax advantageously has a low Noack volatility. In an embodiment, base oil derived from highly paraffinic wax has a Noack volatility between 0 and 100 weight %, and less than the Noack Volatility Factor as calculated by the following equation:

Noack Volatility Factor=160−40(Kinematic Viscosity at 100° C.).

In an embodiment, base oil derived from highly paraffinic wax has a Noack volatility of less than 100 weight %, such as less than 50 weight % or 35 weight %. Accordingly, base oil derived from highly paraffinic wax advantageously has both high viscosity index and low volatility.

In an embodiment, base oil derived from highly paraffinic wax has a viscosity index of greater than 101. In an embodiment, base oil derived from highly paraffinic wax has a viscosity index of between about 105 and about 160.

In an embodiment, the viscosity index of base oil derived from highly paraffinic wax is greater than the Viscosity Index Factor as calculated by the following equation:

Viscosity Index Factor=28×ln(Kinematic Viscosity at 100° C.)+101.

In embodiments, base oil derived from highly paraffinic wax comprises a weight % of molecules with cycloparaffinic functionality of greater than the kinematic viscosity at 100° C. multiplied by three.

In an embodiment, the base oil of the power steering fluid comprises less than 5 weight %, such as less than 1 weight % or less than 0.5 weight %, of unsaturates. In an embodiment, the base oil of the power steering fluid comprises greater than 5 weight % molecules with cycloparaffinic functionality. In an embodiment, the base oil of the power steering fluid comprises a ratio of weight percent of molecules with monocycloparaffinic functionality to weight percent of molecules with multicycloparaffinic functionality of greater than 5, such as greater than 10, greater than 15, or greater than 20. The base oil of the power steering fluid generally comprises less than 0.30 weight percent molecules with aromatic functionality, such as less than 0.1 or less than 0.05 weight percent.

In an embodiment, the base oil of the power steering fluid comprises greater than 5 weight percent molecules with cycloparaffinic functionality. In embodiments, the base oil of the power steering fluid comprises a ratio of weight % of molecules with monocycloparaffinic functionality to weight % of molecules with multicycloparaffinic functionality of greater than 5, such as greater than 10, greater than 15, or greater than 20. In an embodiment, the base oil of the power steering fluid comprises a ratio of weight percent of molecules with cycloparaffinic functionality of greater than the kinematic viscosity at 100° C. multiplied by three.

In some embodiments, the base oil of the power steering fluid comprises less than 19 alkyl branches/100 carbons, for example, greater than 9 alkyl branches/100 carbons and less than 19 alkyl branches/100 carbons. The base oil of the power steering fluid can also have specific alkyl branching placements. In an embodiment, the base oil of the power steering fluid comprises predominantly methyl branching, and the branching is such that there are 6 to 18 alkyl branches per 100 carbons; greater than 25% of the branches are 5 or more carbon atoms apart from each other; and less than 40% of the branches are within 2 to 3 carbon atoms apart from each other. Examples of these types of base oils are taught in U.S. Patent Application Publication No. 2005/0077208 A1.

Base oil containing desired levels of molecules with cycloparaffinic functionality exhibits good solubility for additives, including viscosity index improvers and lubricant additive packages, because molecules with cycloparaffinic functionality impart additive solubility. Base oil containing a high ratio of weight percent of molecules with monocycloparaffinic functionality to weight percent of molecules with multicycloparaffinic functionality (or high weight percent of molecules with monocycloparaffinic functionality and low weight percent of molecules with multicycloparaffinic functionality) are also desirable because molecules with multicycloparaffinic functionality reduce oxidation stability, lower viscosity index, and increase Noack volatility. Accordingly, the base oil exhibits good oxidation stability and high Noack volatility.

In an embodiment, the base oil of the power steering fluid has an aniline point greater than 36×ln(Kinematic Viscosity at 100° C.)+200. Accordingly, the base oil exhibits good elastomer compatibility, and do not damage seals and hoses in power steering systems.

In an embodiment, the base oil of the power steering fluid contains greater than 95 weight % saturates, such as greater than 99 weight % or greater than 99.5 weight %, as determined by elution column chromatography, ASTM D2549-02. Olefins are present in an amount less than detectable by long duration C¹³ Nuclear Magnetic Resonance Spectroscopy (NMR). In an embodiment, molecules with aromatic functionality are present in amounts less than 0.3 weight percent by HPLC-UV, and confirmed by ASTM D5292-99 modified to measure low level aromatics. In an embodiment, molecules with aromatic functionality are present in amounts less than 0.10 weight percent, such as less than 0.05 weight percent, or less than 0.01 weight percent. Sulfur is present in amounts less than 25 ppm, such as 5 ppm, or less than 1 ppm as determined by ultraviolet fluorescence by ASTM D5453-06.

Base oil derived from highly paraffinic wax does not introduce any undesirable characteristics, including, for example, high volatility, high viscosity, and impurities such as heteroatoms, to the power steering fluid. In an embodiment, the base oil is a Fischer-Tropsch derived base oil. Fischer-Tropsch derived waxes are particularly well suited for providing Fischer-Tropsch derived base oil with the above-described properties.

Aromatics Measurement by HPLC-UV:

The method used to measure low levels of molecules with aromatic functionality in the base oils uses a Hewlett Packard 1050 Series Quaternary Gradient High Performance Liquid Chromatography (HPLC) system coupled with a HP 1050 Diode-Array UV-Vis detector interfaced to an HP Chem-station. Identification of the individual aromatic classes in the highly saturated base oils was made on the basis of their UV spectral pattern and their elution time. The amino column used for this analysis differentiates aromatic molecules largely on the basis of their ring-number (or more correctly, double-bond number). Thus, the single ring aromatic containing molecules would elute first, followed by the polycyclic aromatics in order of increasing double bond number per molecule. For aromatics with similar double bond character, those with only alkyl substitution on the ring would elute sooner than those with cycloparaffinic substitution.

Unequivocal identification of the various base oil aromatic hydrocarbons from their UV absorbance spectra was somewhat complicated by the fact their peak electronic transitions were all red-shifted relative to the pure model compound analogs to a degree dependent on the amount of alkyl and cycloparaffinic substitution on the ring system. These bathochromic shifts are well known to be caused by alkyl-group delocalization of the π-electrons in the aromatic ring. Since few unsubstituted aromatic compounds boil in the lubricant range, some degree of red-shift was expected and observed for all of the principle aromatic groups identified.

Quantification of the eluting aromatic compounds was made by integrating chromatograms made from wavelengths optimized for each general class of compounds over the appropriate retention time window for that aromatic. Retention time window limits for each aromatic class were determined by manually evaluating the individual absorbance spectra of eluting compounds at different times and assigning them to the appropriate aromatic class based on their qualitative similarity to model compound absorption spectra. With few exceptions, only five classes of aromatic compounds were observed in highly saturated API Group II and III base oils.

HPLC-UV Calibration:

HPLC-UV is used for identifying these classes of aromatic compounds even at very low levels. Multi-ring aromatics typically absorb 10 to 200 times more strongly than single-ring aromatics. Alkyl-substitution also affected absorption by about 20%. Therefore, it is important to use HPLC to separate and identify the various species of aromatics and know how efficiently they absorb.

Five classes of aromatic compounds were identified. With the exception of a small overlap between the most highly retained alkyl-cycloalkyl-1-ring aromatics and the least highly retained alkyl naphthalenes, all of the aromatic compound classes were baseline resolved. Integration limits for the co-eluting 1-ring and 2-ring aromatics at 272 nm were made by the perpendicular drop method. Wavelength dependent response factors for each general aromatic class were first determined by constructing Beer's Law plots from pure model compound mixtures based on the nearest spectral peak absorbances to the substituted aromatic analogs.

For example, alkyl-cyclohexylbenzene molecules in base oils exhibit a distinct peak absorbance at 272 nm that corresponds to the same (forbidden) transition that unsubstituted tetralin model compounds do at 268 nm. The concentration of alkyl-cycloalkyl-1-ring aromatics in base oil samples was calculated by assuming that its molar absorptivity response factor at 272 nm was approximately equal to tetralin's molar absorptivity at 268 nm, calculated from Beer's law plots. Weight percent concentrations of aromatics were calculated by assuming that the average molecular weight for each aromatic class was approximately equal to the average molecular weight for the whole base oil sample.

This calibration method was further improved by isolating the 1-ring aromatics directly from the base oils via exhaustive HPLC chromatography. Calibrating directly with these aromatics eliminated the assumptions and uncertainties associated with the model compounds. As expected, the isolated aromatic sample had a lower response factor than the model compound because it was more highly substituted.

More specifically, to accurately calibrate the HPLC-UV method, the substituted benzene aromatics were separated from the bulk of the base oil using a Waters semi-preparative HPLC unit. Ten grams of sample was diluted 1:1 in n-hexane and injected onto an amino-bonded silica column, a 5 cm×22.4 mm ID guard, followed by two 25 cm×22.4 mm ID columns of 8-12 micron amino-bonded silica particles, manufactured by Rainin Instruments, Emeryville, Calif., with n-hexane as the mobile phase at a flow rate of 18 mls/min. Column eluent was fractionated based on the detector response from a dual wavelength UV detector set at 265 nm and 295 nm. Saturate fractions were collected until the 265 nm absorbance showed a change of 0.01 absorbance units, which signaled the onset of single ring aromatic elution. A single ring aromatic fraction was collected until the absorbance ratio between 265 nm and 295 nm decreased to 2.0, indicating the onset of two ring aromatic elution. Purification and separation of the single ring aromatic fraction was made by re-chromatographing the monoaromatic fraction away from the “tailing” saturates fraction which resulted from overloading the HPLC column.

This purified aromatic “standard” showed that alkyl substitution decreased the molar absorptivity response factor by about 20% relative to unsubstituted tetralin.

Confirmation of Aromatics by NMR:

The weight percent of molecules with aromatic functionality in the purified mono-aromatic standard was confirmed via long-duration carbon 13 NMR analysis. NMR was easier to calibrate than HPLC UV because it simply measured aromatic carbon so the response did not depend on the class of aromatics being analyzed. The NMR results were translated from % aromatic carbon to % aromatic molecules (to be consistent with HPLC-UV and D 2007) by knowing that 95-99% of the aromatics in highly saturated base oils were single-ring aromatics.

High power, long duration, and good baseline analysis were needed to accurately measure aromatics down to 0.2% aromatic molecules.

More specifically, to accurately measure low levels of all molecules with at least one aromatic function by NMR, the standard D5292-99 method was modified to give a minimum carbon sensitivity of 500:1 (by ASTM standard practice E 386). A 15-hour duration run on a 400-500 MHz NMR with a 10-12 mm Nalorac probe was used. Acorn PC integration software was used to define the shape of the baseline and consistently integrate. The carrier frequency was changed once during the run to avoid artifacts from imaging the aliphatic peak into the aromatic region. By taking spectra on either side of the carrier spectra, the resolution was improved significantly.

Cycloparaffin Distribution by FIMS:

Paraffins are considered more stable than cycloparaffins towards oxidation, and therefore, more desirable. Monocycloparaffins are considered more stable than multicycloparaffins towards oxidation. However, when the weight percent of all molecules with at least one cycloparaffinic function is very low in an oil, the additive solubility is low and the elastomer compatibility is poor. Examples of oils with these properties are Fischer-Tropsch oils with less than about 5% cycloparaffins. To improve these properties in power steering fluids, expensive co-solvents such as esters must often be added. In an embodiment, base oil, derived from highly paraffinic wax and used as dielectric fluids, comprises a high weight percent of molecules with monocycloparaffinic functionality and a low weight percent of molecules with multicycloparaffinic functionality such that the base oil has high oxidation stability, low volatility, good miscibility with other oils, good additive solubility, and good elastomer compatibility.

The base oils of the power steering fluid were characterized by field ionization mass spectroscopy (FIMS) into alkanes and molecules with different numbers of unsaturations. The distribution of molecules in the base oil was determined by FIMS. FIMS spectra were obtained on a Micromass VG 70VSE mass spectrometer. The samples were introduced via a solid probe into the spectrophotometer, for example, by placing a small amount (about 0.1 mg) of the base oil to be tested in a glass capillary tube. The capillary tube was placed at the tip of a solids probe for a mass spectrometer, and the probe was heated from about 40° C. up to 500° C. at a rate of 50° C. per minute, operating under vacuum at approximately 10⁻⁶ Torr. The mass spectrometer was scanned from m/z 40 to m/z 1000 at a rate of 5 seconds per decade. The acquired mass spectra were summed to generate one “averaged” spectrum. Each spectrum was ¹³C corrected using a software package from PC-MassSpec.

Response factors for all compound types were assumed to be 1.0, such that weight percent was determined from area percent. The acquired mass spectra were summed to generate one “averaged” spectrum. The output from the FIMS analysis is the average weight percents of alkanes, 1-unsaturations, 2-unsaturations, 3-unsaturations, 4-unsaturations, 5-unsaturations, and 6-unsaturations in the test sample.

The molecules with different numbers of unsaturations can be comprised of cycloparaffins, olefins, and aromatics. If aromatics were present in significant amounts in the base oil they would most likely be identified in the FIMS analysis as 4-unsaturations. When olefins were present in significant amounts in the base oil they would most likely be identified in the FIMS analysis as 1-unsaturations. The total of the 1-unsaturations, 2-unsaturations, 3-unsaturations, 4-unsaturations, 5-unsaturations, and 6-unsaturations from the FIMS analysis, minus the weight percent of olefins by proton NMR, and minus the weight percent of aromatics by HPLC-UV is the total weight percent of molecules with cycloparaffinic functionality in the base oils of the power steering fluid. The total of the 2-unsaturations, 3-unsaturations, 4-unsaturations, 5-unsaturations, and 6-unsaturations from the FIMS analysis, minus the weight percent of aromatics by HPLC-UV is the weight percent of molecules with multicycloparaffinic functionality in the base oils of the power steering fluid. Note that if the aromatics content was not measured, it was assumed to be less than 0.1 weight % and not included in the calculation for total weight percent of molecules with cycloparaffinic functionality.

In an embodiment, base oil derived from highly paraffinic wax has a weight percent of molecules with cycloparaffinic functionality greater than 5. In an embodiment, base oil derived from highly paraffinic wax also has a high ratio of weight percent of molecules with monocycloparaffinic functionality to weight percent of molecules with multicycloparaffinic functionality, generally greater than 5, greater than 10, greater than 15, or greater than 20.

In an embodiment, there is a relationship between the weight percent of all molecules with at least one cycloparaffinic functionality and the kinematic viscosity of the base oils of the power steering fluid, derived from highly paraffinic wax. That is, the higher the kinematic viscosity at 100° C. in mm²/s, the higher the amount of molecules with cycloparaffinic functionality that are obtained. In an embodiment, base oil derived from highly paraffinic wax has a weight percent of molecules with cycloparaffinic functionality greater than the kinematic viscosity in mm²/s multiplied by three. In an embodiment, base oil derived from highly paraffinic wax has a kinematic viscosity at 100° C. between about 1.2 mm²/s and about 4.0 mm²/s, for example between about 1.2 mm²/s and about 3.5 mm²/s, or between about 2.0 mm²/s and about 3.5 mm²/s, or between about 2.0 mm²/s and about 3.0 mm²/s.

The modified ASTM D5292-99 and HPLC-UV test methods used to measure low level aromatics, and the FIMS test method used to characterize saturates are described in D. C. Kramer, et al., “Influence of Group II & III Base Oil Composition on viscosity index and Oxidation Stability”, presented at the 1999 AIChE Spring National Meeting in Houston, Mar. 16, 1999, the contents of which is incorporated herein in its entirety.

Although the highly paraffinic waxy feeds are essentially free of olefins, base oil processing techniques can introduce olefins, especially at high temperatures, due to ‘cracking’ reactions. In the presence of heat or UV light, olefins can polymerize to form higher molecular weight products that can color the base oil or cause sediment. In general, olefins can be removed by hydrofinishing or by clay treatment.

Additives

The additives for use in base oils to provide power steering fluids include additives selected from the group consisting of viscosity index improvers, pour point depressants, detergents, dispersants, fluidizing agents, friction modifiers, corrosion inhibitors, rust inhibitors, antioxidants, detergents, seal swell agents, antiwear additives, extreme pressure (EP) agents, thickeners, friction modifiers, colorants, dyes, color stabilizers, antifoam agents, corrosion inhibitors, rust inhibitors, seal swell agents, metal deactivators, deodorizers, demulsifiers, anti-squeal agents, and mixtures thereof.

The additives can be in the form of a lubricant additive package, which comprises several additives to provide a power steering fluid with desirable properties. Lubricant additive packages for use in base oils to provide power steering fluids include lubricant additive packages selected from the group consisting of viscosity index improvers, pour point depressants, detergent-inhibitor (DI) additive packages, and mixtures thereof.

I. Viscosity Index Improvers

Viscosity index improvers modify the viscometric characteristics of lubricants by reducing the rate of thinning with increasing temperature and the rate of thickening with low temperatures. Viscosity index improvers thereby provide enhanced performance at low and high temperatures. In many applications, viscosity index improvers are used in combination with detergent-inhibitor additive packages to provide a power steering fluid.

The viscosity index improvers can be selected from the group consisting of olefin copolymers, co-polymers of ethylene and propylene, polyalkylacrylates, polyalkylmethacrylates, styrene esters, polyisobutylene, hydrogenated styrene-isoprene copolymers, star polymers, including those having tetrablock copolymer arms of hydrogenated polyisoprene-polybutadiene-polyisoprene with a block of polystyrene, or hydrogenated asymmetric radial polymers having molecules with a core composed of the remnant of a tetravalant silicon coupling agent, a plurality of rubbery arms comprising polymerized diene units and a block copolymer arm having at least one polymerized diene block and a polymerized monovinyl aromatic compound block, hydrogenated styrene-butadienes, and mixtures thereof. In an embodiment, the viscosity index improver is an ethylene/a-olefin interpolymer as described in WO 2006/102146 A2, wherein the ethylene/a-olefin interpolymer is a block copolymer having at least a hard block and at least a soft block. The soft block comprises a higher amount of comonomers than the hard block. In an embodiment, the viscosity index improver is an acrylic acid ester polymer comprising a copolymer derived from a first acrylic acid ester monomer having from about 1 to about 4 carbon atoms, a second acrylic acid ester monomer having from about 12 to about 14 carbon atoms, and a third acrylic acid ester monomer having from about 16 to about 20 carbon atoms, as described in U S. Patent Application Publication No. 2006/0252660 A1, wherein the copolymer has weight average molecular weight of 20,000-100,000 daltons and contains 1 weight % or less of unreacted monomer.

II. Pour Point Depressants

Pour point depressants used in power steering fluids modify the wax crystal morphology such as to reduce interlocking of the wax crystals with consequent viscosity increase or gellation. Examples of pour point depressants are alkylated naphthalene and phenolic polymers, polymethacrylates, alkylated bicyclic aromatics, maleate/fumarate copolymer esters, methacrylate-vinyl pyrrolidone copolymers, styrene esters, polyfumerates, vinyl acetate-fumarate co-polymers, dialkyl esters of phthalate acid, ethylene vinyl acetate copolymers, and other mixed hydrocarbon polymers from commercial additive suppliers such as LUBRIZOL, the ETHYL Corporation, or ROHMAX, a Division of Degussa.

III. Pour Point Reducing Blend Component

In some embodiments a base oil pour point reducing blend component can be used. As used herein, “pour point reducing blend component” refers to an isomerized waxy product with relatively high molecular weights and a specified degree of alkyl branching in the molecule, such that it reduces the pour point of lubricating base oil blends containing it. Examples of a pour point reducing blend component are disclosed in U.S. Pat. Nos. 6,150,577 and 7,053,254, and U.S. Patent Application Publication No. US 2005-0247600 A1. A pour point reducing blend component can be: 1) an isomerized Fischer-Tropsch derived bottoms product; 2) a bottoms product prepared from an isomerized highly waxy mineral oil, or 3) an isomerized oil having a kinematic viscosity at 100° C. of at least about 8 mm²/s made from polyethylene plastic.

In one embodiment, the pour point reducing blend component is an isomerized Fischer-Tropsch derived vacuum distillation bottoms product having an average molecular weight between 600 and 1100 and an average degree of branching in the molecules between 6.5 and 10 alkyl branches per 100 carbon atoms. Generally, the higher molecular weight hydrocarbons are more effective as pour point reducing blend components than the lower molecular weight hydrocarbons. In one embodiment, a higher cut point in a vacuum distillation unit which results in a higher boiling bottoms material is used to prepare the pour point reducing blend component. The higher cut point also has the advantage of resulting in a higher yield of the distillate base oil fractions. In one embodiment, the pour point reducing blend component is an isomerized Fischer-Tropsch derived vacuum distillation bottoms product having a pour point that is at least 3° C. higher than the pour point of the distillate base oil it is blended with.

In one embodiment, the 10 percent point of the boiling range of the pour point reducing blend component that is a vacuum distillation bottoms product is between about 850-1050° F. (454-565° C.). In another embodiment, the pour point reducing blend component is derived from either Fischer-Tropsch or petroleum products, having a boiling range above 950° F. (510° C.), and contains at least 50 percent by weight of paraffins. In yet another embodiment the pour point reducing blend component has a boiling range above 1050° F. (565° C.).

In another embodiment, the pour point reducing blend component is an isomerized petroleum derived base oil containing material having a boiling range above about 1050° F. In one embodiment, the isomerized bottoms material is solvent dewaxed prior to being used as a pour point reducing blend component. The waxy product further separated during solvent dewaxing from the pour point reducing blend component were found to display excellent improved pour point depressing properties compared to the oily product recovered after the solvent dewaxing.

In another embodiment, the pour point reducing blend component is an isomerized oil having a kinematic viscosity at 100° C. of at least about 8 mm²/s made from polyethylene plastic. In one embodiment the pour point reducing blend component is made from waste plastic. In another embodiment the pour point reducing blend component is made from steps comprising pyrolysis of polyethylene plastic, separating out a heavy fraction, hydrotreating the heavy fraction, catalytic isomerizing the hydrotreated heavy fraction, and collecting the pour point reducing blend component having a kinematic viscosity at 100° C. of at least about 8 mm²/s. In a third embodiment, the pour point reducing blend component derived from polyethylene plastic and has a boiling range above 1050° F. (565° C.), or even has a boiling range above 1200° F. (649° C.).

In one embodiment, the pour point reducing blend component has an average degree of branching in the molecules within the range of from 6.5 to 10 alkyl branches per 100 carbon atoms. In another embodiment, the pour point reducing blend component has an average molecular weight between 600-1100. In a third embodiment it has an average molecular weight between 700-1000. In one embodiment, the pour point reducing blend component has a kinematic viscosity at 100° C. of 8-30 mm²/s, with the 10% point of the boiling range of the bottoms falling between about 850-1050° F. In yet another embodiment, the pour point reducing blend component has a kinematic viscosity at 100° C. of 15-20 mm²/s and a pour point of −8 to −12° C.

In one embodiment, the pour point reducing blend component is an isomerized oil having a kinematic viscosity at 100° C. of at least about 8 mm²/s made from polyethylene plastic. In one embodiment the pour point reducing blend component is made from waste plastic. In another embodiment the pour point reducing blend component is made from steps comprising pyrolysis of polyethylene plastic, separating out a heavy fraction, hydrotreating the heavy fraction, catalytic isomerizing the hydrotreated heavy fraction, and collecting the pour point reducing blend component having a kinematic viscosity at 100° C. of at least about 8 mm²/s. In a third embodiment, the pour point reducing blend component derived from polyethylene plastic has a boiling range above 1050° F. (565° C.), or even a boiling range above 1200° F. (649° C.).

IV. Detergent-Inhibitor Additive Packages

Detergent-inhibitor additive packages serve to suspend oil contaminants, as well as to prevent oxidation of the power steering fluids with the resultant formation of varnish and sludge deposits. The detergent-inhibitor additive package useful in power steering fluids contains one or more conventional additives selected from the group consisting of dispersants, fluidizing agents, friction modifiers, corrosion inhibitors, rust inhibitors, antioxidants, detergents, seal swell agents, extreme pressure additives, antiwear additives, deodorizers, antifoam agents, demulsifiers, colorants, and color stabilizers. The detergent-inhibitor additive package is present in an amount of from 2 to 25 weight percent, based on the total weight of the power steering fluid composition. Detergent-inhibitor additive packages are readily available from additive suppliers such as LUBRIZOL, ETHYL, Oronite, and INFINEUM. A number of detergent-inhibitor additives are described in EP 0 978 555 A1.

V. Dispersants

Dispersants are used in power steering fluids to disperse wear debris and products of lubricant degradation within the equipment being lubricated (i.e., power steering equipment). The ashless dispersants commonly used contain a lipophilic hydrocarbon group and a polar functional hydrophilic group. The polar functional group can be of the class of carboxylate, ester, amine, amide, imine, imide, hydroxyl, ether, epoxide, phosphorus, ester carboxyl, anhydride, or nitrile. The lipophilic group can be oligomeric or polymeric in nature, usually from 70 to 200 carbon atoms to ensure good oil solubility. Hydrocarbon polymers treated with various reagents to introduce polar functions include products prepared by treating polyolefins such as polyisobutene first with maleic anhydride, or phosphorus sulfide or chloride, or by thermal treatment, and then with reagents such as polyamine, amine, ethylene oxide, etc.

Of these ashless dispersants the ones typically used in power steering fluids include N-substituted polyisobutenyl succinimides and succinates, alkyl methacrylate-vinyl pyrrolidinone copolymers, alkyl methacrylate-dialkylaminoethyl methacrylate copolymers, alkylmethacrylate-polyethylene glycol methacrylate copolymers, and polystearamides. Some oil-based dispersants that are used in power steering fluids include dispersants from the chemical classes of alkylsuccinimide, succinate esters, high molecular weight amines, and Mannich base and phosphoric acid derivatives. Some specific examples are polyisobutenyl succinimide-polyethylenepolyamine, polyisobutenyl succinic ester, polyisobutenyl hydroxybenzyl-polyethylenepolyamine, bis-hydroxypropyl phosphorate. Commercial dispersants suitable for power steering fluid are for example, LUBRIZOL 890 (an ashless PIB succinimide), LUBRIZOL 6420 (a high molecular weight PIB succinimide), and ETHYL HITEC 646 (a non-boronated PIB succinimide). The dispersant can be combined with other additives used in the lubricant industry to form an additive package for power steering fluid, e.g., LUBRIZOL 9677MX, and the whole additive package can be used as the dispersing agent.

Alternatively a surfactant or a mixture of surfactants with low HLB value (typically less than or equal to 8), for example, nonionic, or a mixture of nonionics and ionics, can be used as the dispersants in the power steering fluid.

The dispersants selected should be soluble or dispersible in the liquid medium or additive diluent oil. The dispersant can be in a range of up from 0.01 to 30 percent and all sub-ranges therebetween, for example in a range of from between 0.5 percent to 20 percent, a range of from between 1 to 15 percent, or in a range of from between 2 to 13 percent as active ingredient in the power steering fluid.

VI. Fluidizing Agents

Fluidizing agents are sometimes used in power steering fluids. Suitable fluidizing agents include oil-soluble diesters. Examples of diesters include the adipates, azelates, and sebacates of C₈-C₁₃ alkanols (or mixtures thereof), and the phthalates of C₄-C₁₃ alkanols (or mixtures thereof). Mixtures of two or more different types of diesters (e.g., dialkyl adipates and dialkyl azelates, etc.) can also be used. Examples of such materials include the n-octyl, 2-ethylhexyl, isodecyl, and tridecyl diesters of adipic acid, azelaic acid, and sebacic acid, and the n-butyi, isobutyl, pentyl, hexyl, heptyl, octyl, nonyl, decyl, undecyl, dodecyl, and tridecyl diesters of phthalic acid. Other esters which are used as fluidizing agents in power steering fluids are polyol esters such as EMERY 2918, 2939 and 2995 esters from the EMERY Group of Henkel Corporation and HATCOL 2926, 2970 and 2999.

VII. Thickeners

Other thickeners, besides viscosity index improvers, which can be used in the power steering fluid include: acrylic polymers such as polyacrylic acid and sodium polyacrylate, high-molecular-weight polymers of ethylene oxide such as Polyox WSR from Union Carbide, cellulose compounds such as carboxymethylcellulose, polyvinyl alcohol (PVA), polyvinyl pyrrolidone (PVP), xanthan gums and guar gums, polysaccharides, alkanolamides, amine salts of polyamide such as DISPARLON AQ series from King Industries, hydrophobically modified ethylene oxide urethane (e.g., ACRYSOL series from Rohmax), silicates, and fillers such as mica, silicas, cellulose, wood flour, clays (including organoclays) and clays, and resin polymers such as polyvinyl butyral resins, polyurethane resins, acrylic resins and epoxy resins.

Other examples of thickeners are polyisobutylene, high molecular weight complex ester, butyl rubber, olefin copolymers, styrene-diene polymer, polymethacrylate, styrene-ester, and ultra high viscosity PAO. An example of a high molecular weight complex ester is Priolube® 3986. To achieve thickening and also impart low traction coefficient properties an ultra high viscosity PAO can also be used in the formulation. As used herein, an “ultra high viscosity PAO” has a kinematic viscosity between about 150 and 1,000 mm²/s or higher at 100° C.

VIII. Friction Modifiers

Friction modifiers are optionally used in power steering fluids. Suitable friction modifiers include such compounds as aliphatic amines or ethoxylated aliphatic amines, aliphatic fatty acid amides, aliphatic carboxylic acids, aliphatic carboxylic esters, aliphatic carboxylic ester-amides, aliphatic phosphonates, aliphatic phosphates, aliphatic thiophosphonates, aliphatic thiophosphates, or mixtures thereof. The aliphatic group typically contains at least about eight carbon atoms so as to render the compound suitably oil soluble. Also suitable are aliphatic substituted succinimides formed by reacting one or more aliphatic succinic acids or anhydrides with ammonia.

One group of friction modifiers is comprised of the N-aliphatic hydrocarbyl-substituted diethanol amines in which the N-aliphatic hydrocarbyl-substituent is at least one straight chain aliphatic hydrocarbyl group free of acetylenic unsaturation and having in the range of about 14 to about 20 carbon atoms.

Another group of friction modifiers is comprised of esters of fatty acids, for example CENWAX™ TGA-185 and glycerol esters of selected fatty acids such as UNIFLEX™ 1803, both made by Arizona Chemical. Other fatty acids used as friction modifiers are mono-oleates such as glycerol mono-oleate, pentaerythritol mono-oleate, and sorbitan mono-oleate sold under the tradename of RADIASURF™ by OLEON.

Friction modifiers will sometimes include a combination of at least one N-aliphatic hydrocarbyl-substituted diethanol amine and at least one N-aliphatic hydrocarbyl-substituted trimethylene diamine in which the N-aliphatic hydrocarbyl-substituent is at least one straight chain aliphatic hydrocarbyl group free of acetylenic unsaturation and having in the range of about 14 to about 20 carbon atoms. Further details concerning this friction modifier combination are set forth in U.S. Pat. Nos. 5.372,735 and 5,441,656.

Another example of a mixture of friction modifiers is based on the combination of (i) at least one di(hydroxyalkyl) aliphatic tertiary amine in which the hydroxyalkyl groups, being the same or different, each contain from 2 to about 4 carbon atoms, and in which the aliphatic group is an acyclic hydrocarbyl group containing from about 10 to about 25 carbon atoms, and (ii) at least one hydroxyalkyl aliphatic imidazoline in which the hydroxyalkyl group contains from 2 to about 4 carbon atoms, and in which the aliphatic group is an acyclic hydrocarbyl group containing from about 10 to about 25 carbon atoms. Further details concerning this friction modifier system are found in U.S. Pat. No. 5,344,579.

Another class of friction modifiers that is sometimes used in power steering fluids include compounds of the formula: in which Z is a group R1R2CH—, in which R1 and R2 are each independently straight- or branched-chain hydrocarbon groups containing from 1 to 34 carbon atoms and the total number of carbon atoms in the groups R1 and R2 is from 11 to 35. The radical Z is, for example, 1-methylpentadecyl, 1-propyltridecenyl, 1-pentyltridecenyl, 1-tridecenylpentadecenyl or 1-tetradecyleicosenyl. These compounds are commercially available or are made by the application or adaptation of known techniques (see, for example, EP 0 020 037 A1 and U.S. Pat. Nos. 5,021,176, 5,190,680, and RE-34,459).

The use of friction modifiers is optional. However, in applications where friction modifiers are used, the power steering fluid will contain up to about 1.25 weight %, such as from about 0.05 to about 1 weight % of one or more friction modifiers.

IX. Corrosion Inhibitors

Corrosion inhibitors are another class of additives suitable for inclusion in power steering fluids. Such compounds include thiazoles, triazoles and thiadiazoles. Examples of such compounds include benzotriazole, tolyltriazole, octyltriazole, decyltriazole, dodecyltriazole, 2-mercapto benzothiazole, 2,5-dimercapto-1,3,4-thiadiazole, 2-mercapto-5-hydrocarbylthio-1,3,4-thiadiazoles, 2-mercapto-5-hydrocarbyldithio-1,3,4-thiadiazoles, 2,5-bis(hydrocarbylthio)-1,3,4-thiadiazoles, and 2,5-bis(hydrocarbyldithio)-1,3,4-thiadiazoles. Corrosion inhibitors of these types that are available on the open market include Cobratec TT-100 and HITEC® 314 additive and HITEC® 4313 additive (ETHYL Petroleum Additives, Inc.).

X. Rust Inhibitors

Rust inhibitors comprise another type of inhibitor additive. Some rust inhibitors are also corrosion inhibitors. Examples of rust inhibitors useful in power steering fluids are monocarboxylic acids and polycarboxylic acids. Examples of suitable monocarboxylic acids are octanoic acid, decanoic acid and dodecanoic acid. Suitable polycarboxylic acids include dimer and trimer acids such as are produced from such acids as tall oil fatty acids, oleic acid, linoleic acid, or the like. Products of this type are currently available from various commercial sources, such as, for example, the dimer and trimer acids sold under the HYSTRENE trademark by the Humko Chemical Division of Witco Chemical Corporation and under the EMPOL trademark by Henkel Corporation. Another useful type of rust inhibitor for use in power steering fluids is comprised of the alkenyl succinic acid and alkenyl succinic anhydride corrosion inhibitors such as, for example, tetrapropenylsuccinic acid, tetrapropenylsuccinic anhydride, tetradecenylsuccinic acid, tetradecenylsuccinic anhydride, hexadecenylsuccinic acid, hexadecenylsuccinic anhydride, and the like. Also useful are the half esters of alkenyl succinic acids having 8 to 24 carbon atoms in the alkenyl group with alcohols such as the polyglycols. Another suitable rust inhibitor is a rust inhibitor comprising a solubility improver having an aniline point less than 100° C.; a mixture of amine phosphates; and an alkenyl succinic compound selected from the group consisting of an acid half ester, an anhydride, an acid, and mixtures thereof, as taught in U.S. patent application Ser. No. 11/257,900, filed on Oct. 25, 2005. Other suitable rust or corrosion inhibitors include ether amines; acid phosphates; amines: polyethoxylated compounds such as ethoxylated amines, ethoxylated phenols, and ethoxylated alcohols; imidazolines; aminosuccinic acids or derivatives thereof, and the like. Materials of these types are available as articles of commerce. Mixtures of rust inhibitors can be used.

XI. Antioxidants

Suitable antioxidants include phenolic antioxidants, aromatic amine antioxidants, sulfurized phenolic antioxidants, hindered phenolic antioxidants, molybdenum containing compounds, zinc dialkyldithiophosphates, and organic phosphites, among others. Mixtures of different types of antioxidants are often used. Examples of phenolic antioxidants include ionol derived hindered phenols, 2,6-di-tert-butylphenol, liquid mixtures of tertiary butylated phenols, 2,6-di-tert-butyl-4-methylphenol, 4,4′-methylenebis(2,6-di-tert-butylphenol), 2,2′-methylenebis(4-methyl-6-tert-butylphenol), mixed methylene-bridged polyalkyl phenols, 4,4′-thiobis(2-methyl-6-tert-butylphenol), and sterically hindered tertiary butylated phenols. N,N′-di-sec-butyl-p-phenylenediamine, 4-isopropylaminodiphenyl amine, phenyl-naphthyl amine, phenyl-naphthyl amine, styrenated diphenylamine, and ring-alkylated diphenylamines serve as examples of aromatic amine antioxidants. In an embodiment, the antioxidant is a catalytic antioxidant comprising one or more oil soluble organo metallic compound(s) and/or organo metallic coordination complexes such as metal(s) or metal cation(s) having more than one oxidation state above the ground state complexed, bonded or associated with two or more anions, one or more bidentate or tridentate ligands and/or two or more anions and ligand(s), as described in U.S. Patent Application Publication No. 2006/0258549 A1.

XII. Detergents

Examples of detergents that can be used in power steering fluids are over-based metallic detergents, such as the phosphonate, sulfonate, phenolate or salicylate types as described in Kirk-Othmer Encyclopedia of Chemical Technology, third edition, volume 14, pages 477-526.

XIII. Seal Swell Agents

A number of seal swell agents useful in power steering fluids are described in U.S. Patent Application Publication Nos. 2003/0119682 A1 and 2007/0057226 A1. Examples of seal swell agents are aryl esters, long chain alkyl ether, alkyl esters, vegetable based esters, sebacate esters, sulfolanes, substituted sulfolane, other sulfolane derivatives, phenates, adipates, glyceryl tri(acetoxystearate), epoxidized soybean oil, epoxidized linseed oil, N, n-butyl benzene sulfonamide, aliphatic polyurethane, polyester glutarate, triethylene glycol caprate/caprylate, dialkyl diester glutarate, monomeric, polymer, and epoxy plasticizers, phthalate plasticizers, such as dioctyl phthalate, dinonly phthalate or dihexylpthalate, or oxygen-, sulfur-, or nitrogen-containing polyfunctional nitriles, phenates, and combinations thereof. Other plasticizers which can be substituted for and/or used with the above plasticizers including glycerine, polyethylene glycol, dibutyl phthalate, and 2,2,4-trimethyl-1,3-pentanediol monoisobutyrate, and diisononyl phthalate all of which are soluble in a solvent carrier. Other seal swelling agents such as LUBRIZOL 730 can also be used.

XIV. Antiwear and/or Extreme Pressure Additives

Various types of sulfur-containing antiwear and/or extreme pressure additives can be used in power steering fluids. Examples include dihydrocarbyl polysulfides; sulfurized olefins; sulfurized fatty acid esters of both natural and synthetic origins; trithiones; sulfurized thienyl derivatives; sulfurized terpenes: sulfurized oligomers of C₂-C₈ monoolefins; and sulfurized Diels-Alder adducts such as those disclosed in U.S. Pat. No. RE-27,331. Specific examples include sulfurized polyisobutene, sulfurized isobutylene, sulfurized diisobutylene, sulfurized triisobutylene, dicyclohexyl polysulfide, diphenyl polysulfide, dibenzyl polysulfide, dinonyl polysulfide, and mixtures of di-tert-butyl polysulfide such as mixtures of di-tert-butyl trisulfide, di-tert-butyl tetrasulfide and di-tert-butyl pentasulfide, among others. Combinations of such categories of sulfur-containing antiwear and/or extreme pressure agents can also be used, such as a combination of sulfurized isobutylene and di-tert-butyl trisulfide, a combination of sulfurized isobutylene and dinonyl trisulfide, a combination of sulfurized tall oil and dibenzyl polysulfide.

In the context of this disclosure a component which contains both phosphorus and sulfur in its chemical structure is deemed a phosphorus-containing antiwear and/or extreme pressure agent rather than a sulfur-containing antiwear and/or extreme pressure agent.

Use can be made of a wide variety of phosphorus-containing oil-soluble antiwear and/or extreme pressure additives such as the oil-soluble organic phosphates, organic phosphites, organic phosphonates, organic phosphonites, etc., and their sulfur analogs. Also useful as the phosphorus-containing antiwear and/or extreme pressure additives that can be used in power steering fluids include those compounds that contain both phosphorus and nitrogen. Phosphorus-containing oil-soluble antiwear and/or extreme pressure additives useful in power steering fluids include those compounds taught in U.S. Pat. Nos. 5,464,549, 5,500,140, and 5,573,696.

One such type of phosphorus- and nitrogen-containing antiwear and/or extreme pressure additives which can be used in power steering fluids are the phosphorus- and nitrogen-containing compositions of the type described in GB 1,009,913, GB 1,009,914, U.S. Pat. No. 3,197,405 and/or U.S. Pat. No. 3,197,496. In general, these compositions are formed by forming an acidic intermediate by the reaction of a hydroxy-substituted triester of a phosphorothioic acid with an inorganic phosphorus acid, phosphorus oxide or phosphorus halide, and neutralizing a substantial portion of said acidic intermediate with an amine or hydroxy-substituted amine. Other types of phosphorus- and nitrogen-containing antiwear and/or extreme pressure additive that can be used in power steering fluids include the amine salts of hydroxy-substituted phosphetanes or the amine salts of hydroxy-substituted thiophosphetanes and the amine salts of partial esters of phosphoric and thiophosphoric acids.

XV. Antifoam Agents

Antifoam agents work by destabilizing the liquid film that surrounds entrained air bubbles. To be effective they must spread effectively at the air/liquid interface. According to theory, the antifoam agent will spread if the value of the spreading coefficient, S, is positive. S is defined by the following equation:

S=P ¹ −P ² −P ^(1,2),

wherein P¹ is the surface tension of the foamy liquid, P² is the surface tension of the antifoam agent, and P^(1,2) is the interfacial tension between them. Surface tension and interfacial tensions are measured using a ring type tensiometer by ASTM D1331-89 (Reapproved 2001), “Standard Test Methods for Surface and Interfacial Tension of Solutions of Surface-Active Agents”. With respect to the present disclosure, P¹ is the surface of the power steering fluid prior to the addition of antifoam agent.

Examples of antifoam agents are antifoam agents that when blended into the power steering fluid will exhibit spreading coefficients of at least 2 mN/m at both 24° C. and 93.5° C. Various types of antifoam agents are taught in U.S. Pat. No. 6,090,758. When used, the antifoam agents should not significantly increase the air release time of the power steering fluid. Examples of suitable antifoam agents are high molecular weight polydimethyl siloxane, a type of silicone antifoam agent, acrylate antifoam agents (as they are less likely to adversely effect air release properties compared to lower molecular weight silicone antifoam agents), polydimethylsiloxanes and polyethylene glycol ethers and esters.

XVI. Colorants/Dyes

Colorants or dyes are used to impart color or to fluoresce under particular types of light. Fluorescent dyes facilitate leak detection. Colored oils help distinguish between different products. Examples of these colorants or dyes are anthraquinones, azo compounds, triphenyl-methane, perylene dye, naphthalimide dye, and mixtures thereof. Particular types of fluorescent dyes are taught in U.S. Pat. No. 6,165,384.

XVII. Diluent Oil

Diluent oil is often used in the different types of additive packages to effectively suspend or dissolve the additives in a liquid medium. In general, the maximum amount of diluent oil in all of the additive packages used to make the power steering fluid should be within 0 to 40 volume %. In an embodiment, the diluent oil is an extra light hydrocarbon liquid derived from highly paraffinic wax, described in U.S. Patent Application Publication No. 2006/0201852 A1, wherein the diluent oil has a viscosity of between about 1.0 mm²/s and about 3.5 mm²/s at 100° C. and a Noack volatility of less than 50 weight %, and also having greater than 3 weight % molecules with cycloparaffinic functionality and less than 0.30 weight percent aromatics.

EXAMPLES

The following illustrative examples are intended to be non-limiting.

Example 1 Equation (2)

As disclosed in U.S. Patent Application Publication No. 2006/0201852 A1, a sample of commercial hydrotreated Fischer-Tropsch wax made using a Fe-based Fischer-Tropsch synthesis catalyst and a sample of hydrotreated Fischer-Tropsch wax made using a Co-based Fischer-Tropsch catalyst were analyzed and found to have the properties shown in Table I.

TABLE I Fischer-Tropsch Catalyst Fe-Based Co-Based Sulfur, ppm <6 Nitrogen, ppm 6, 5* Oxygen by NA, Wt % 0.59 GC N-Paraffin Analysis Total N Paraffin, Wt % 84.47 Avg. Carbon Number 27.3 Avg. Molecular Weight 384.9 D 6352 Sim. Dist. (Wt %), ° F.  0.5 10 515  5 131 597 10 181 639 20 251 689 30 309 714 40 377 751 50 437 774 60 497 807 70 553 839 80 611 870 90 674 911 95 707 935 99.5 744 978 *duplicate tests

The Fischer-Tropsch wax feeds were hydroisomerized over a Pt/SAPO-11 catalyst on an alumina binder. Run conditions were a temperature of between 652 and 695° F. (344 and 368° C.), liquid hourly space velocity (LHSV) of 0.6 to 1.0 hr⁻¹, 1000 psig reactor pressure, and a once-through hydrogen rate of between 6 and 7 MSCF/bbl. The reactor effluent passed directly to a second reactor, also at 1000 psig, which contained a Pt/Pd on silica-alumina hydrofinishing catalyst. Conditions in that reactor were a temperature of between 425 and 700° F. (218 and 372° C.), and LHSV of 1.0 hr⁻¹.

The products boiling above about 600° F. were fractionated by atmospheric or vacuum distillation to produce five fractions having viscosities between about 2.0 and 3.5 mm²/s at 100° C. The properties of the five fractions are shown in Table II.

TABLE II Properties FT-1 FT-2 FT-3 FT-4 FT-5 Wax Feed Fe- Fe- Co- Co- Co- Based Based Based Based Based Hydroisomerization Temp, ° F. 681 681 694 671 690 Viscosity at 100° C., mm²/s 2.981 2.598 2.583 2.297 3.189 VI 127 124 133 124 122 Aromatics, Wt % 0.0128 0.0107 FIMS, Wt % of Molecules Paraffins 89.2 91.1 93.0 91.3 81.3 Monocycloparaffins 10.8 8.9 7.0 8.0 18.7 Multicycloparaffins 0.0 0.0 0.0 0.7 0.0 Total 100.0 100.0 100.0 100.0 100.0 API Gravity 43.4 44.1 43.85 44.69 Pour Point, ° C. −27 −32 −30 −33 5 Cloud Point, ° C. −18 −22 −16 −7 12 Mono/Multicycloparaffins >100 >100 >100 11.4 >100 Oxidator BN, Hours 40.14 Aniline Point, D 611-04, ° F. 236.5 226.3 Noack Volatility, Wt % 32.48 49.18 48.94 21.8 Noack Volatility Factor 40.76 56.08 56.68 68.12 32.44 According to Equation (2): 160-40(Viscosity at 100° C.) D 6352 Sim. Dist. (Wt %), ° F.  0.5 652 597 601 591 672  5 670 615 618 605 695 10 681 626 630 616 707 20 697 646 653 634 722 30 713 666 673 652 734 40 728 686 693 668 744 50 744 706 713 684 755 60 760 726 733 699 767 70 776 748 754 715 779 80 792 769 777 732 793 90 808 791 802 750 810 95 817 803 816 767 823 95.5 833 825 833 800 850 NMR, Alkyl Branches/100 10.05 10.36 Not 9.46 9.20 Carbons tested

The Noack volatility of four of the fractions, FT-1, FT-2, FT-3, and FT-5, were each less than an amount calculated by the equation:

Noack Volatility Factor=160−40(Kinematic Viscosity at 100° C.).   (2)

Example 2 Equation (3)

As disclosed in U.S. patent application Ser. No. 11/613,936, three samples of Fischer-Tropsch derived base oil were analyzed and determined to have the following properties:

TABLE III Properties FT-A FT-B FT-C Kin. Vis @ 40° C., mm²/s 10.00 10.85 11.76 Kin Vis @ 100° C., mm²/s 2.806 2.926 3.081 VI 130 124 124 Pour Point, ° C. −40 −37 −43 Noack Volatility, Wt % 34.32 32.37 27.23 CCS Vis @−40° C., mPa · s <900 1238 1398 D 6352 SIMDIST TBP (WT %), ° F. 0.5/5 655/672 665/683 677/695 10/30 681/705 692/717 704/727 50 727 737 747 70/90 747/772 755/777 765/787 95 782 785 795 Wt % Aromatics 0.0063 0.0131 0.0043 Wt % Olefins <0.05 <0.05 <0.05 Oxidator BN, Hours 59.56 40.16 39.09 Noack Volatility Factor by 35.07 29.53 23.54 Equation (3): 900 × (KV100)^(−2.8) − 15

The three Fischer-Tropsch derived base oils were all distillate fractions made by hydroisomerization dewaxing a hydrotreated Co-based Fischer-Tropsch wax in a series of two reactors, hydrofinishing the effluent in a single reactor, and vacuum distilling the product into different grades of base oil. All three of these Fischer-Tropsch derived base oils had very low aromatics and olefin contents, and had very good oxidation stabilities. Additionally, all three of them had very low Noack volatilities. Note that only the FT-A had a wt % Noack Volatility less than an amount defined by the equation:

Noack Volatility Factor=(900×(Kinematic Viscosity at 100° C.)^(−2.8))−15.   (3)

The difference between the wt % Noack volatility of the light base oil fraction FT-A and the Noack Volatility Factor by Equation (3) of FT-A was greater than 0.5. FT-A also had extremely good oxidation stability and a viscosity index greater than:

28×ln(Kinematic Viscosity at 100° C.)+95.

Example 3 Equation (3)

As further disclosed in U.S. patent application Ser. No. 11/613,936, hydrotreated Co-based Fischer-Tropsch wax was hydroisomerized over a Pt/SAPO-11 hydroisomerization catalyst in a series of three reactors at a temperature of 600-700° F., about 1 LHSV feed rate, less than 800 psig pressure, and about 4 to about 20 MSCF/bbl hydrogen flow rate. Following hydroisomerization, the product was hydrofinished over a Pd/Silica Alumina hydrofinishing catalyst in a series of two hydrofinishing reactors at a total pressure greater than 700 psig, a temperature of about 400 to about 600° F., about 1 LHSV feed rate, and about 4 to about 20 MSCF/bbl hydrogen flow rate.

The products out of the hydrofinishing reactor were vacuum distilled into different base oil grades, one or more fractions having a kinematic viscosity at 100° C. between 1.5 and 3.5 mm²/s. Two of these base oil fractions were analyzed and determined to have the following properties:

TABLE IV Properties FT-D FT-E Kin Vis @ 100° C., mm²/s 1.768 2.919 VI 126 Pour Point, ° C. −57 −31 Noack Volatility, Wt % 82.13 22.5 D6352 SIMDIST TBP (WT %), ° F. 0.5/5 148/443 672/693 10/30 546/615 702/721 50 645 737 70/90 669/693 754/777 95 702 788 Wt % Aromatics 0.0174 <0.005 Wt % Olefins <0.05 0.11 Oxidator BN, Hours 49.92 64.04 Noack Volatility Factor by Equation (3): 167.5 29.8 900 × (KV100)^(−2.8) − 15

Both of these base oils had a wt % Noack volatility between 0 and 100 and additionally less than an amount defined by the equation:

Noack Volatility Factor=(900×(Kinematic Viscosity at 100° C.)^(−2.8))−15.   (3)

The difference between the wt % Noack volatilities of the light base oil fractions FT-D and FT-E and their Noack Volatility Factors by Equation (3) were greater than 5. They both had exceptionally good oxidation stabilities, low pour points, and high VIs.

Example 4 Power Steering Fluids

Two base oils were prepared by hydroisomerization dewaxing a Co-based Fischer-Tropsch wax and a Fe-based Fischer-Tropsch wax over a Pt/SAPO-11 catalyst at 1000 psi, 0.5-1.5 LHSV, and between 660-690° C. The base oils were subsequently hydrotreated to reduce the level of aromatics and olefins, then vacuum distilled into fractions.

The FIMS analysis was conducted on a Micromass Time-of-Flight spectrophotometer. The emitter on the Micromass Time-of-Flight was a Carbotec 5 um emitter designed for FI operation. A constant flow of pentaflourochlorobenzene, used as lock mass, was delivered into the mass spectrometer via a thin capillary tube. The probe was heated from about 50° C. up to 600° C. at a rate of 100° C. per minute. Test data on the two Fischer-Tropsch derived lubricant base oils (i.e., Base Oil 1, appropriate for use in the presently claimed power steering fluids, as well as a Comparative Base Oil) are shown in Table V, below.

TABLE V Comparative Base Oil 1 Base Oil Made from Co-based Fischer- Fe-based Tropsch wax Fischer- Tropsch wax Hydroisomerization Temp, ° F. 600-750 600-750 Hydroisomerization Dewaxing Pt/SAPO-11 Pt/SAPO-11 Catalyst Hydrogen to Feed Ratio, MSCF/bbl 6-7 6-7 Viscosity at 40° C., mm²/s 6.939 11.05 Viscosity at 100° C., mm²/s 2.18 2.981 Viscosity Index 123 127 API Gravity 44.99 43.4 Pour Point, ° C. −37 −27 TGA Noack Volatility, wt % 67.4 48.0 Aromatics by HPLC-UV, wt % <0.1 0.0128 FIMS Analysis, wt % Alkanes 93.2 89.2 1-Unsaturation 6.8 10.8 2- to 6-Unsaturations 0.0 0.0 Wt % Olefins <1.0 0.9 Average Molecular Weight 324 357 Wt % Molecules with >5.8 9.9 Monocycloparaffinic Functionality Wt % Molecules with 0.00 0.00 Multicycloparaffinic Functionality Ratio of Monocycloparaffins/ >100 >100 Multicycloparaffins Noack Volatility Factor as calculated 72.8 40.76 by Equation (2) Noack Volatility Factor as calculated 86.52 27.27 by Equation (3) X in the equation: 101 96 VI = 28 × ln(Viscosity at 100° C.) + X

Base Oil 1, unlike the Comparative Base Oil, has a kinematic viscosity at 100° C. greater than a Viscosity Index Factor calculated by the following equation:

28×ln(Kinematic Viscosity at 100° C.)+101.   (1)

Base Oil 1, unlike the Comparative Base Oil, also has a Noack volatility less than a Noack Volatility Factor calculated by either of the following equations:

160−40(Kinematic Viscosity at 100° C.).   (2)

(900×(Kinematic Viscosity at 100° C.)^(−2.8))−15.   (3)

As noted above, for kinematic viscosities in the range of 2.4 to 3.8 mm²/s, Equation (3) provides a lower Noack Volatility Factor than does Equation (2). As the kinematic viscosity of Base Oil 1 is 2.18 mm²/s, Equation (3) does not provide a lower Noack Volatility Factor (86.52) than does Equation (2) (72.8) for Base Oil 1. However, as the kinematic viscosity of the Comparative Base Oil is 2.981 mm²/s, Equation (3) does provide a lower Noack Volatility Factor (27.27) than does Equation (2) (40.76) for the Comparative Base Oil. However, the TGA Noack Volatility of the Comparative Base Oil, 48 wt %, is greater than either 27.27 or 40.76, while the TGA Noack Volatility of Base Oil 1, 67.37 wt %, is less than either 86.52 or 72.8.

TABLE VI Comparative Comparative Power Power Power HC (Fortum) Steering Steering Steering Current Fluid A Fluid B Fluid Formulation Base Oil Base Base Oil 1 Comparative Fortum Oil 1 Base Oil Nexbase 3043 Base Oil, wt % 84.45 84.75 84.45 84.75 Viscosity Index 11 11 10 11 Improver, wt % Detergent-Inhibitor 4.25 4.25 4.25 4.25 Additive Package, wt % Pour Point Depressant, 0.3 — 0.3 — wt % Kinematic Viscosity at 5.57 5.46 6.68 6.24 100° C., mm²/s Kinematic Viscosity at 17.12 16.69 23.72 20.64 40° C., mm²/s Viscosity Index 312 313 266 288 Brookfield Viscosity at 1400 1270 6800 1900 −40° C., mPa · s

Fortum Nexbase 3043 is a conventional API Group III base oil. The total 1-to 6- unsaturations by FIMS for Fortum Nexbase 3043 is greater than 55 weight %, and the ratio of molecules with monocycloparaffinic functionality to molecules with multicycloparaffinic functionality is less than 2.0.

All of the publications, patents and patent applications cited herein are herein incorporated by reference in their entirety to the same extent as if the disclosure of each individual publication, patent application or patent was specifically and individually indicated to be incorporated by reference in its entirety.

Many modifications of the exemplary embodiments disclosed herein will readily occur to those of skill in the art. Accordingly, the present disclosure is to be construed as including all structure and methods that fall within the scope of the appended claims. 

1. A power steering fluid comprising: a) greater than 50 weight % base oil, wherein the base oil has consecutive numbers of carbon atoms and has a viscosity index greater than a Viscosity Index Factor calculated by the following equation: 28×ln(Kinematic Viscosity at 100° C.)+101; and b) viscosity index improver; wherein the power steering fluid has a viscosity index of greater than 290 and a Brookfield Viscosity at −40° C. of less than 1900 mPa·s.
 2. The power steering fluid of claim 1, further comprising less than about 1.0 weight % pour point depressant.
 3. The power steering fluid of claim 1, wherein the base oil is made from a waxy feed.
 4. The power steering fluid of claim 1, wherein the base oil has a kinematic viscosity at 100° C. between about 1.2 and about 4.0 mm²/s.
 5. The power steering fluid of claim 1, wherein the base oil has a kinematic viscosity at 100° C. between about 1.5 and about 3.5 mm²/s.
 6. The power steering fluid of claim 1, wherein the base oil has greater than 5.0 weight % molecules with cycloparaffinic functionality.
 7. The power steering fluid of claim 1, wherein the base oil has a ratio of molecules with monocycloparaffinic functionality to molecules with multicycloparaffinic functionality of greater than
 5. 8. The power steering fluid of claim 1, wherein the base oil has a ratio of molecules with monocycloparaffinic functionality to molecules with multicycloparaffinic functionality of greater than
 10. 9. The power steering fluid of claim 1, wherein the base oil has a ratio of molecules with monocycloparaffinic functionality to molecules with multicycloparaffinic functionality of greater than
 15. 10. The power steering fluid of claim 1, wherein the base oil has a ratio of molecules with monocycloparaffinic functionality to molecules with multicycloparaffinic functionality of greater than
 20. 11. The power steering fluid of claim 1, comprising greater than 70 weight % base oil.
 12. The power steering fluid of claim 1, comprising viscosity index improver in an amount less than 13 weight %.
 13. The power steering fluid of claim 1, comprising viscosity index improver in an amount less than 12 weight %.
 14. The power steering fluid of claim 1, wherein the power steering fluid comprises approximately 0 weight % pour point depressant.
 15. The power steering fluid of claim 1, wherein the Brookfield Viscosity at −40° C. is less than 1500 mPa·s.
 16. The power steering fluid of claim 1, further comprising a detergent-inhibitor additive package.
 17. The power steering fluid of claim 16, comprising 2 to 6 weight % detergent-inhibitor additive package.
 18. The power steering fluid of claim 16, comprising less than 5 weight % detergent-inhibitor additive package.
 19. The power steering fluid of claim 1, wherein the base oil is Fischer-Tropsch derived.
 20. The power steering fluid of claim 1, wherein the power steering fluid meets the requirements of one or more power steering fluid specifications for automotive power steering systems.
 21. A process for producing the power steering fluid of claim 1 comprising: a) obtaining a base oil having: i) a consecutive numbers of carbon atoms; ii) a viscosity index greater than a Viscosity Index Factor calculated by the following equation: 28×ln(Kinematic Viscosity at 100° C.)+101; and b) blending the base oil with viscosity index improver to form the power steering fluid; wherein the power steering fluid comprises greater than 50 weight % base oil and has a viscosity index of greater than 290 and a Brookfield Viscosity at −40° C. of less than 1900 mPa·s.
 22. A process for producing a power steering fluid comprising: a) obtaining a base oil having: i) a consecutive numbers of carbon atoms; ii) a kinematic viscosity at 100° C. of less than about 4 mm²/s; and iii) a Noack volatility less than a Noack Volatility Factor calculated by the following equation: 160−40(Kinematic Viscosity at 100° C.); and b) blending the base oil with viscosity index improver to form the power steering fluid; wherein the power steering fluid comprises greater than 50 weight % base oil and has a viscosity index of greater than 290 and a Brookfield Viscosity at −40° C. of less than 1900 mPa·s.
 23. The process of claim 22, wherein forming a power steering fluid further comprises blending the base oil with pour point depressant, wherein the power steering fluid comprises less than about 1.0 weight % pour point depressant.
 24. The process of claim 22, wherein the base oil has a viscosity index greater than a Viscosity Index Factor calculated by the following equation: 28×ln(Kinematic Viscosity at 100° C.)+101.
 25. The process of claim 22, wherein the base oil has a kinematic viscosity at 100° C. between about 1.2 and about 4.0 mm²/s.
 26. The process of claim 22, wherein the base oil has a kinematic viscosity at 100° C. between about 1.5 and about 3.5 mm²/s.
 27. The process of claim 22, wherein the base oil is Fischer-Tropsch derived.
 28. The process of claim 22, wherein the base oil has greater than 5.0 weight % molecules with cycloparaffinic functionality.
 29. The process of claim 22, wherein the base oil has a ratio of molecules with monocycloparaffinic functionality to molecules with multicycloparaffinic functionality of greater than
 5. 30. The process of claim 22, wherein the base oil has a ratio of molecules with monocycloparaffinic functionality to molecules with multicycloparaffinic functionality of greater than
 10. 31. The process of claim 22, wherein the base oil has a ratio of molecules with monocycloparaffinic functionality to molecules with multicycloparaffinic functionality of greater than
 15. 32. The process of claim 22, wherein the base oil has a ratio of molecules with monocycloparaffinic functionality to molecules with multicycloparaffinic functionality of greater than
 20. 33. The process of claim 22, wherein forming a power steering fluid further comprises blending the base oil with a detergent-inhibitor additive package.
 34. The process of claim 33, wherein the detergent-inhibitor additive package is blended in an amount of 2 to 6 weight %.
 35. The process of claim 33, wherein the detergent-inhibitor additive package is blended in an amount of less than 5 weight %.
 36. The process of claim 22, wherein the base oil has an aniline point greater than 36×ln(Kinematic Viscosity at 100° C.)+200.
 37. The process of claim 22, wherein the base oil has a Noack volatility between 0 and
 100. 38. The process of claim 37, wherein the base oil has a Noack volatility less than a Noack Volatility Factor calculated by the following equation: 900×(Kinematic Viscosity at 100° C.)^(−2.8)−15.
 39. A power steering fluid produced by the process of claim 22 comprising: a) greater than 50 weight % base oil, wherein the base oil has consecutive numbers of carbon atoms, a kinematic viscosity at 100° C. of less than about 4 mm²/s, and a Noack volatility less than a Noack Volatility Factor calculated by the following equation: 160−40(Kinematic Viscosity at 100° C.); and b) viscosity index improver; wherein the power steering fluid has a viscosity index of greater than 290 and a Brookfield Viscosity at −40° C. of less than 1900 mPa·s. 